This invention relates to a pipe joint for use with oil well pipes used in the exploration and production of natural gas and crude oil. In particular, it relates to a pipe joint which reduces galling of threads, which has good operability at the time of tightening, and which reduces the level of thread damage which sometimes occurs thereby.
Threaded joints are widely used as a technology for connecting oil well pipes used in the exploration and production of natural gas and crude oil. Threaded joints include integral types in which a pin portion is formed on one end of a pipe and a box portion is formed on the other end and two pipes are directly connected to each other, and coupling types in which two pipes having a pin portion formed on each end are connected to each other by a short pipe having a box portion formed at both ends. Below, a description will be given using a coupling-type joint as an example, but the same applies to an integral-type joint.
At the site of many gas fields and oil fields, tightening (make-up) of a joint is carried out as shown in FIG. 7.
Thus, an oil well pipe 2 having a short pipe 1 previously fastened to its end in a factory is supported with the short pipe 1 facing upwards, and the pin portion 2b of another oil well pipe 2 which is to be connected thereto is fastened to the short pipe 1. In this case, when the pin portion 2b of the oil well pipe 2 which is to be connected is inserted into the box portion 1a of the short pipe 1, the oil well pipe 2 is inserted vertically into the short pipe 1 through a plastic stabbing guide (not shown), for example, which is a jig for preventing damage at the time of mating engagement which is provided in advance on the periphery of the short pipe 1.
After pipe insertion is completed, the stabbing guide is removed, and the oil well pipe 2 which is to be connected is rotated two to three turns by a strap wrench or by the strength of the operator to achieve a state in which the thread of the pin portion 2b of the oil well pipe 2 which is to be connected engages with the thread of the box portion 1a of the short pipe 1. Then, a prescribed torque is applied with power tongs, namely, with a torque-applying apparatus (not shown) to perform tightening.
Hand tightening which is carried out by an operator is rotation with a small torque (on the order of 19.6-576 Nxc2x7m) which is only on the level of human strength. However, when the threads of the pin portion and the box portion are not completely engaged with each other, rotation will not take place with a torque on the order of 576 Nxc2x7m, and a higher torque than that is necessary for rotation. This means that the state of engagement between the threads of the pin portion and the box portion is defective. In such a case, the operator performs fine adjustment by slightly raising the oil well pipe to be connected or slightly rotating it in the opposite direction to correct the misalignment. When fine adjustment is completed, it becomes possible to carry out rotation with the threads in engagement using a very small torque of at most 576 Nxc2x7m, so the oil well pipe to be connected is rotated two to three turns in the tightening direction.
However, recently, off shore (in oil fields on the sea) and the like, there has developed a demand for tightening which does not require rotation of an oil well pipe by two to three turns by hand tightening by an operator after an oil well pipe is inserted using a stabbing guide. Thus, as shown in FIG. 8, automatic power tongs 3 grasp an oil well pipe 2 to be connected, and in this state, tightening corresponding to conventional hand tightening and power tightening are continuously carried out.
Hands-free power tongs of this type have a hydraulic drive apparatus (torque generating apparatus) and can easily apply a torque of 33810 Nxc2x7m to an oil well pipe having an outer diameter on the order of 178 mm.
However, it is difficult to control the movement of such power tongs with applying a minute torque at a level of 576 Nxc2x7m and it is difficult to carry out careful hand tightening on the order of 576 Nxc2x7m which has been performed by an operator. Thus, tightening operation is always performed with a strong force, and when the threads are not completely engaged at the time of pipe insertion, damage to the threads may occur in some cases.
In the prior art, in JP H11-223284A, for example, a pipe joint has been proposed with which after the pin portion of an oil well pipe which is to be connected is inserted into the box portion of a short pipe, it can be screwed in as quickly as possible and with a small number of turns. This pipe joint is premised on it being ensured that the pin portion of the oil well pipe is inserted rightly in a vertical direction into the box portion of the short pipe using a stabbing guide or the like.
In addition, in the above-described Japanese patent publication, the stabbing relief angle xcex2 of the thread of the pin portion of the oil well pipe is sloped at 3 degrees. However, in the case of a buttress thread shape for a thread prescribed by API for oil well pipes (at least 16 inches) (406.4 mm)), from in the past, in order to improve thread engagement, the stabbing relief angle xcex2 has been prescribed as being parallel to the vertical, i.e., the stabbing relief angle equals 0 degrees. According to this view, theoretically, the engagement of the threads of the pin portion of an oil well pipe and of the box portion of a short pipe is guaranteed. Accordingly, if the stabbing relief angle xcex2 is made a daring angle, the thread height becomes small, and this leads to a decrease in the portion which receives axial force.
Furthermore, in the above-described Japanese patent publication, in order to improve engagement between threads at the time of pipe insertion, it is proposed to perform marking of the threads of the pin portion of an oil well pipe and the box portion of a short pipe in a location where it is easiest to perform engagement. Although it is not impossible for an operator to perform tightening until the marks are aligned, when it is dark such as at night, it is difficult to ascertain the marks, so this cannot necessarily be described as suitable technology.
Due to the introduction of new technology, when an oil well pipe is being lowered into an oil well, it is necessary to investigate various underground data (the temperature, pressure, depth, and the like), and for this purpose, there are cases in which a wire line (a connecting line for a sensor) is attached. In this case, in the conventional method in which a well is dug vertically as shown in FIG. 9(a), if the oil well pipe 2 swings near the vicinity of its axis, there is a great danger of the wire line 4 being severed.
In order to perform lowering of an oil well pipe into the ground without breaking the wire line 4, as shown in FIG. 9(b), a technique is being developed in which an oil well is dug at an angle of 1-1.5 degrees with respect to the vertical.
However, when employing such a technique, it is necessary for hand tightening and subsequent power tightening to be carried out in a state in which peripheral equipment is also tilted. There is no problem in performing hand tightening or power tightening if all equipment is maintained in a state in which it is tilted by 1-1.5 degrees with respect to the vertical, but there is a high likelihood of power tongs being horizontally installed, so at the time of hand tightening or power tightening, there is the possibility of tightening being carried out as if bending were being applied, and pipe joints come up against severe tightening conditions.
However, in the past, there have not been any proposals for remedying such a problem.
This invention was made in light of the above-described problems, and its object is to provide a pipe joint which can reduce galling of threads, with which tightening operations can be carried out in a satisfactory manner, and which can reduce the level of damage to threads which sometimes occurs, even when a pin portion which is to be connected to a box portion is not inserted in place reliably or when it is inserted inadequately, as encountered in the case in which an oil well is drilled with a tilt of 1-2 degrees, for example.
Another object of this invention is to provide a pipe joint which can reduce the level of damage to threads when tightening corresponding to hand tightening and power tightening is continuously carried out with power tongs even in the case in which a pin portion which is to be connected to a box portion is not inserted in place reliably or when it is inadequately inserted.
In the past, when a pin portion to be connected to a box portion was not reliably inserted in place or when it was inadequately inserted, fine engagement was carried out by the operator after pipe insertion. However, when there is the premise that such hand tightening by an operator is not carried out, in order to reduce galling of threads and achieve satisfactory ease of tightening, it is necessary to modify the shape of the joint itself so as to make fine engagement by hand tightening unnecessary.
When a pin portion is inserted in place vertically with respect to a box portion in which each portion has a tapered thread, engagement of the tapered male thread of the pin portion and the tapered female thread of the box portion can be roughly classified into the states shown in FIGS. 10(a)-(c). Of these, in the case shown in FIG. 10(c), the tapered male thread of the pin portion 2b and the tapered female thread of the box portion 1a are completely engaged with each other, and it is necessary to rotate them one to two further turns in order to carry out hand tightening with certainty, but it is not necessary to perform rotation in order to engage them with each other.
On the other hand, in the case of FIGS. 10(a) and (b), in order to go from these incompletely engaged states to the desirable engaged state shown in FIG. 10(c), it is necessary to perform rotation by one turn or 0.5 turns.
It is normally not easy to perform pipe insertion in place, i.e., to achieve the desirable engaged state shown in FIG. 10(c). Therefore, regardless which of the engaged states shown in FIGS. 10(a)-(c) occurs, it is desired for the pin portion to be able to rotate with a small resistance under a very small rotational force on the level of hand tightening (on the order of 576 Nxc2x7m).
The present invention was made based on the results of various investigations with respect to the effect of thread shape on the ease of tightening at the time of pipe insertion and the amount of damage to threads. Thus, according to the present invention, in a pipe joint in which a pin portion having a tapered male thread formed on the outer peripheral surface of the end portion of a pipe is screwed into a box portion having a tapered female thread formed on the inner peripheral surface of the end of a pipe or a short pipe so as to threadingly engage with the tapered male thread, the clearance C between the stab flanks of the threads is made 0.5-1.0 mm, the stab flank angle xcex8 of the threads is made 15-60 degrees, the thread crest surface and the thread root surfaces of the threadingly engaged tapered male thread and tapered female thread are made parallel to the pipe axis, the radius of curvature R of the rounded portions of the stab flank corners of the male thread (refer to below as the xe2x80x9cmale thread stab flank corner portionsxe2x80x9d) is made 1.0-1.25 mm, and preferably the taper of an incomplete thread portion of the pin portion is made 0.96-0.90 times the nominal taper of a complete thread portion.
Thus, according to the present invention, even when a pin portion is not reliably inserted in place in a box portion or when it is not adequately inserted therein, whether tightening with power tongs is carried out after rotation by hand tightening or whether tightening is carried out by rotation with hands-free power tongs, it is made possible to decrease galling of threaded portions and perform tightening satisfactorily with decreasing the level of damage to threads which sometimes occurs during tightening.
According to the present invention, it is unnecessary to perform fine adjustment for realignment and subsequent adjustment of engagement by rotating 1-0.5 turns.